1. Field of the Invention
Generally, the present invention relates to a golf ball and more particularly to a dimple pattern of the golf ball.
2. Description of the Related Art
Normally, the golf ball has 280-540 dimples on its surface. The role of the dimple lies in accelerating transition of turbulent flows of a boundary layer by making air flows in the periphery of the golf ball turbulent to thereby cause separation of the turbulent air flows (hereinafter referred to as “effect of turbulent flow”) while it is flying. The acceleration of the transition of the turbulent air flows allows air to be separated from a rear point of the golf ball and a pressure drag to be low to thereby increase the flight distance thereof. Owing to the acceleration of the transition of the turbulent air flows, it is possible to space the separation point at upper side of the golf ball away from that at the lower side thereof and thereby improve a lift acting on the golf ball. Therefore, it can be said that a dimple pattern capable of accelerating the transition of the turbulent air flows, namely, capable of making the air flow turbulent is aerodynamically superior.
Researches and investigations have been made on the dimple pattern. For example, in Japanese Patent Publication No. 57-22595, there is disclosed a golf ball having the flat portion (region other than dimple-forming region) with a predetermined area formed on its surface. Because the flat portion is present on the surface of the golf ball, the golf ball has a comparatively small region covered with the dimples on its surface. This, the effect of a turbulent flow is displayed insufficiently and thus the golf ball is aerodynamically unsatisfactory.
There are proposed golf balls having dimples formed on their surfaces as densely as possible to enhance their flight performances. For example, in Japanese Patent Application Laid-Open No. 64-8982, there is disclosed a golf ball having a surface area occupancy percentage (a value obtained by dividing the total sum of the area of all dimples formed on the golf ball by the surface area of an imaginary sphere having a diameter equal to that of the golf ball) at 65% or more. As another example, in Japanese Patent Application Laid-Open No. 9-347177, there is disclosed a golf ball having 40 flat portions or less in which rectangles each having an area larger than a predetermined area is formed.
Normally, the golf ball is shaped by using a pair of semispherical upper and lower dies. In molding a material of the golf ball, a part thereof flows out from a seam (parting line) between the upper and lower half dies. As a result, an annular burr is formed on; the surface of the golf ball. The burr is abraded with a whetstone or the like to remove it from the surface of the golf ball after the molding process terminates. Normally, no dimples are formed on the parting line to allow a region to be abraded to be flat to thereby abrade the region easily. Consequently, a great circle is formed on the portion (hereinafter referred to as “seam”) corresponding to the parting line on the golf ball. The annular region containing the great circle constitutes a great circle zone not intersecting with dimples.
The above-described golf ball on which dimples are arranged densely is superior in its flight performance. Thus, a golfer can send the golf ball flying a long distance. However, golfers; desire to obtain a longer flight. Thus, golf ball makers are demanded to produce a golf ball having a longer flight. In the golf ball on which dimples are arranged densely, there is a big difference between the density of dimple arranged in the great circle zone and the density of dimples arranged in the region other than the great circle zone. Therefore, the golf ball does not look fine and its aerodynamic symmetrical property is insufficient.